Liquid scintillation counting is a method for determining the amount of one or more radioactive substances. The method is used predominently for determining beta-radioactive isotopes, such as .sup.3 H, .sup.14 C and .sup.32 P.
Beta-radiative isotopes decay by emitting energy in the form of a fast electron and a neutrino. The energy liberated in the decay is always constant for a certain radioisotope, but is divided between the electron and the neutrino according to a distribution law. The neutrino can not be detected by using liquid scintillation counting but the electron will through collisional impact, transfer some of its energy to the liquid solvent molecules which are then ionized or excited to higher energy levels. Provided that the solvent molecules are predominantly of aromatic character and that certain fluorizing compounds are dissolved in the solution, part of the excitation energy deposited by the electron may cause an emission of photons which can be detected by a photosensitive device such as a photomultiplier. The intensity of the light pulse caused by a decay is proportional to the energy of the electron when ejected from the nucleus. The height of the electrical pulse measured at the output of the photomultiplier is again proportional to the number of photons in the light pulse. As each decay produces one distinct pulse, with a height proportional to the energy of the beta electron, a certain pulse height distribution or spectrum, can be recorded. The shape of this pulse distribution depends not only on the decay characteristics but also on the efficiency of the liquid to transform excitation energy into light and the efficiency of the detector to transform photons into detectable electrical pulses.
FIG. 1 on the accompanying drawing shows typical pulse height distributions (spectra) for .sup.3 H and .sup.14 C, measured in a liquid scintillation counter having two photomultipliers working in coincidence and a multi-channel analyzer. The number of pulses in the pulse height distribution detected per time unit is called the count rate.
Quenching of the scintillation light pulse means that the number of photons produced in a decay, where the electron has a certain energy, is diminished. Hence, quenching results generally in both lower pulse heights and lower count rates. As the object in most measurements is to determine the activity, i.e. the disintegration rate, and not only the count rate, the relation between activity and count rate must be known. This relation is equal to the counting efficiency of the sample. As the counting efficiency may vary from sample to sample even within one measurement batch, it becomes necessary to determine the counting efficiency for each sample.
The determination of the efficiency of an unknown sample relies on calibration of the instrument. This step includes the measurement of a number of calibration samples containing known amounts of the pure radioisotopes to be measured and having different levels of quench. For each radioisotope one such quench calibration set must include at least two calibration samples. Each quench set thus results in a quench calibration function, giving counting efficiency as a function of some quench index, e.g. end point of external standard spectrum. In the case of two calibration samples for each radioisotope, the quench function will be a straight line. The quench function provides means to interpolate between, and to some extent extrapolate from, the calibration sample points.
As one unknown sample may contain several radioisotopes, the counter must have means for distinguishing the contribution of each radioisotope and also for determining the activity of each radioisotope. One such multi-labeled sample may further have a quench level not equal to any of the calibration samples. Generally the spectra of each radioisotope overlaps one another more or less (as apparent from FIG. 1). This provides for a complicated situation especially in the case when the sample contains more than two isotopes.
W. L. Oller and P. Plato (International Journal of Applied Radiation and Isotopes, 1972, Vol. 23, 481-485), indicate a method combining least squares fit and spectral analysis for determining the activities of all radioisotopes in a multi-labelled liquid scintillation sample. In the method of Oller and Plato, a beta spectrum of each sample was recorded and analyzed by using "a least squares spectrum analysis computer program". Oller and Plato do not give any details on how their program works and what it is based on. Neither do they take quench into consideration, as all unknowns are supposed to be of the same quench level.